1. Field of the Invention
The present invention relates to an imaging apparatus using the Talbot interference method.
2. Description of the Related Art
The Talbot interference method is a method of obtaining a phase image of an object using interference of electromagnetic waves such as X-ray and light.
The principle of the Talbot interference method will be described. An imaging apparatus including an electromagnetic wave source, diffraction grating, shield grating, and detector is commonly used for the Talbot interference method. If an electromagnetic wave emitted from the electromagnetic wave source passes through an object, the phase of the electromagnetic wave changes depending on the refractive index and shape of the object. If the electromagnetic wave is diffracted by the diffraction grating, an interference pattern having phase information of the object is formed at specific positions called Talbot positions. By detecting the interference pattern using the detector and analyzing detection results, a differential phase image of the object can be obtained.
However, depending on a size of pitch of an interference pattern or a resolution of a detector, it may be difficult for the detector to detect the interference pattern. In such a case, a shiegrating is arranged at positions where the interference pattern is formed. The shield grating includes a shield portion that blocks electromagnetic waves and a transmission portion through which electromagnetic waves pass and forms a moire by shielding a portion of an interference pattern. A phase image of an object is obtained by detecting intensity information of the moire using a detector and analyzing the detection result.
An analysis method for obtaining a phase image of an object from a detection result of the detector using a Fourier transform is known. The analysis method will briefly be described. First, a Fourier transform of a detection result is performed to obtain a spatial frequency spectrum. Next, the spectrum of frequency of a fundamental period component (hereinafter referred to as the carrier frequency) of a moire and peripheral frequencies thereof are cut out and moved to the origin. A differential phase image of the object is obtained by performing an inverse Fourier transform of the frequency spectrum and further, a phase image of the object is obtained by integrating the differential phase image. In an imaging apparatus using the Talbot interference method, as described above, the shield grating is used to shield a first portion of the interference pattern and allow a second portion thereof to pass through.
Japanese Patent Application Laid-Open No. 2007-203064 (corresponding version: U.S. Pat. No. 7,564,941) discusses a shield grating used for a one-dimensional interference pattern in the Talbot interference method using X-ray (hereinafter referred to as the X-ray Talbot interference method). The arrangement of the shield portion and transmission portion of the shield grating corresponds to that of a bright portion and a dark portion of the interference pattern and the ratio of the width of the transmission portion to that of the shield portion is 1:1.
Generally, high intensity of the carrier frequency makes the differential phase image less susceptible to noise. Thus, to obtain a phase image of an object using the Talbot interference method, high intensity of the carrier frequency is desirable. However, the intensity of the carrier frequency is not sufficient in an imaging apparatus using the conventional two-dimensional Talbot interference method.